Fluorine-containing compounds are employed for various uses. To be specific, for instance, it is known that fluoromethane (CH3F), trifluoromethane (CHF3) and pentafluoroethane (CF3CHF2) are used for e.g., refrigerants and material gas for semiconductors, and carbonyl difluoride (COF2) is used for e.g., etching gas for semiconductors and cleaning gas for semiconductors.
To achieve a purity required for these uses, fluorine-containing compound purification methods are known to separate a fluorine-containing compound that contains impurities into a fluorine-containing compound and impurities.
For instance, as a carbonyl difluoride purification method, methods described in the following Patent documents 1 to 3 are known.
Patent document 1 discloses a method for removing silicon tetrafluoride by bringing carbonyl difluoride that contains silicon tetrafluoride as an impurity into contact with a metal fluoride. The silicon tetrafluoride is hardly by-produced in a usual carbonyl difluoride production method.
Patent document 2 discloses a method for separating a mixed gas into carbon dioxide and carbonyl difluoride by introducing the mixed gas that contains carbon dioxide and carbonyl difluoride, each of which has a boiling point close to each other, to a film separation apparatus.
Patent document 3 discloses a method for separating a mixture that contains carbonyl difluoride, obtained through a reaction between an oxygen-containing compound and a fluorine gas, and trifluoromethylhypofluorite as an impurity, into carbonyl difluoride and trifluoromethylhypofluorite by using an activated carbon.
Furthermore, it is also known that a fluorine-containing compound that contains hydrogen chloride as an impurity (a crude fluorine-containing compound), is difficult to separate.
To separate such a crude fluorine-containing compound into a fluorine-containing compound and hydrogen chloride, it has been known to add an aqueous solvent, e.g., an alkali aqueous solution and water, to the crude fluorine-containing compound so as to dissolve hydrogen chloride in the aqueous solvent and thereby separate hydrogen chloride dissolved in the aqueous solvent from the fluorine-containing compound, which does not readily dissolve relatively in the aqueous solvent.